Antimonide coated magnetic materials with lead and lead-antimony matrices



2,999,778 ANTIMONIDE COATED MAGNETIC MATERIALS WITH LEAD AND LEAD-ANQNY MATRICES Lewis I. Mendelsohn, Swampscott, Mass, assignor to General Electric Company, a corporation of New York No Drawing. Filed Dec. 16, 1957, er. No. 702,803 Claims. (Cl. 148-3155) This invention relates to improved magnetic materials comprising elongated ultra-fine magnetic particles and to the process of manufacture of such materials.

The manufacture of elongated magnetic particles having transverse dimensions which are those of a single magnetic domain by plating a magnetic material into a .molten metal cathode is described in patent application Serial No. 500,078, filed April 8, 1955, now Patent 2,974,104, March 7, 1961, assiged to the same assignee as this invention. By maintaining the interface between the electrolyte and the liquid cathode in an undisturbed or quiescent condition, elongated particles of magnetic material having a median elongation ratio'of at least 1.5 to 1 and having at least one-half of the particles possessed of an elongation ratio of at least 2 to 1 are produced. This represents a significant advance over the prior art which produced largely spheriodal or relatively blunt magnetic particles. In the above process, the magnetic particles are removed from the molten metal cathode such as liquid mercury by means of a permanent magnet dipped into the cathode, the magnetic material being extracted as a putty-like mass of fine magnetic particles and mercury. The particle-mercury slurry is then heated for a few minutes at about 200 C. and after cool-' ing to room temperature, a trace of a non-magnetic metal such as tin is added. Gross removal of the magnetic particles from the mercury is continued by oxidizing the iron with air and the resulting powder. is then washed and vacuum or hydrogen baked at a low temperature to eliminate the last traces of mercury. A non-magnetic metal filler or a non-metallic filler is then mixed with the elongate-:1 magnetic particles, the particles being aligned by a magnetic field and the mixture pressed into ,a final firm magnetic structure. Further details of this process will be found in the above referred to patent which is -hereby incorporated by reference in this application.

The magnetic-particles as plated have a dendritic structure and the heat treatment above described reduces the branch-like structure and increases the iron particle diameter, thus increasing the coercive force of the iron or other magnetic material substantially. The addition of a material such as tin, zinc, aluminum, manganese, nickel, antimony or other metal which compounds with the magnetic material to the slurry plates the elongatedarticles separating them with a non-magnetic material. According to the above application, the last traces of mercury can be removed by washing the metal coated, elongated particles by adding a layer of lead alloy such as lead antimony alloy to the slurry, thus reducing the mercury concentration and removing the cluster of elongated particles from the slurry with a permanent magnet. These steps of dilution and removal of the magnetic material are repeated until the mercury concentration is at the point desired. The last traces of mercury can also be removed by heating the slurry of mercury and metal coated elongated particles at a temperature of about 250 C. for about 3 hours under a vacuum of about 1 micron of mercury.

While the useofoxidation techniques to remove the mercury or other molten metal matrix results in the effective removal of the iron and produces a magnetic material suitable for many purposes, it isinherently accompanied' by substantial losses of magnetic saturation inductiontB s) and residual induction (B with substanran Patented Sept. 12, 2981 or potassium chromate, etc. also leaves up to 30 percent of mercury or molten metal matrix with the oxidized iron. Metal washing techniques for removal of the major part of the mercury, as well as the last'traces of mercury using low melting point alloys were explored, but these alloys are either too expensive for mass production purposes or produce poor physical properties. Vacuum distillation procedures, while successful in removing mercury, are operative at relatively 'high temperatures of above about 250 C. and cause the elongated particles to spheroidize at the high temperatures and results in poor coercive forces and lower magnetic energies in general.

In order to retain the magnetic properties of the magnetic material at distillation or washing processing temperatures above 250 C., it was necessary that the magnetic material particles be stabilized by some means at such temperatures.

According to copendingapplication Serial No. 702,801,

filed December 16, 1957, assigned to the same assignee as this invention, finely divided magnetic particles may be stabilized in the absence of oxidizing influences up to temperatures of 400 C. by treating them with antimony to form an antimonide coating on the magnetic particles. In the search for a suitable binder or matrix material which would serve to bind the magnetic particles together in the final magnetic structure and protect them from oxidation, various alloys of lead and antimony were used and notably the eutectic lead-antimony alloy containing 10 percent antimony and percent lead. However, it was found that, using the lead-antimony eutectic, even when the mercury was distilled or washed oil at 250 C., the antimonide coating permeated entirely into the iron to form an antimonide of the complete particle, thus destroying its permanent magnet qualities. Since the antimony dissolved into the iron, destroying its magnetic qualities even at 250 C. for the eutecticalloy, it appeared quite obvious that the antimonide coating would not remain stable at the melting point of lead or 327 C. if pure lead were used.

- A principal object of this invention is to provide a matrix or binderv for finely divided antimony-treated magnetic particles which is non-reactive therewith and will protect them at elevated temperatures, as well as magnetic structures derived therefrom.

It was unexpectedly found, according to this invention,

despite all evidence to the contrary, that pure lead or lead containing up to a maximum of about 2 percent or less by weight of antimony, whenv used as a matrix or binder for antimonide-coated particles such as of iron, iron-cobalt, etc., provided a non-reactive carrier for the magnetic particles, and served to protect the particles from the effects of oxidation. In addition to protecting the coated magnetic particles against chemical reaction, the lead or lead alloy as described provides a suitable physical spacer or carrier for the particles. Furthermore, the magnetic particles have good temperature stability in mixtures or solutions of lead or lead alloys containing up to about 2 percent of antimony in mercury.

Those features of the invention which are believed to be novel are set forth with particularity in the claims appended hereto. The invention will, however, be more fully understood and further advantages thereof appreciatedfrom a consideration of the following description.

The lead (or lead-antimony alloy prescribed) may be added to the mixture of magnetic particles and mercury in any desired manner. For example, it may be added as elemental lead in the form of chunks or pellets or as mixtures of lead and mercury, a 50-50 mixture of lead and mercury being particularly easy to blend into a uniform mixture with the iron-antimony-mercury containing slurry. The amount of lead is critical only in so far as that a minimum amount of about 50 grams of lead per 1000 grams of iron, antimony, and mercury slurry should be added to protect the iron from oxidation when the mercury has been removed. This is equivalent to about 1 gram of lead for each gram of iron. Quantities in excess of the above amount of lead may, of course, be used, but these affect only the so-called packing factor or magnetic particle concentration of the finished magnetic structure or article. When lead is used as the matrix material, it has also been found convenient to form the antimonide on the magnetic particles after the addition of the lead rather than before, although either procedure, of course, may be used. The amounts of antimony to be added to the magnetic particle mercury slurry and other processing details are more fully set forth and claimed in copcnding application Serial No. 702,801, filed December 16, 1957, assigned to the same assignee as this invention and incorporated herein by reference.

The amount of antimony which can betolerated in the lead matrix material is critical. Shown in Table I below is the weight percent concentration of antimony in the lead or lead-mercury alloy, along with the percentage of body centered cubic type of iron or magnetic particles present, along with the coercive force corresponding to these concentrations after an exposure of 30 minutes to a temperature of 350 C.

From the above, it will be evident that only up to about 2 percent and preferably about 1.5 percent by weight of antimony can be tolerated in the matrix material if severe magnetic degradation is not to take place.

Coated magnetic particles having a matrix of lead or lead-antimony as prescribed herein can be readily processed at relatively high temperatures. For example, mercury can be washed or removed by dilution or distilled off at temperatures up to 350 C. without detracting essentially from the magnetic characteristics of the particles.

Before distilling or washing the mercury from the slurry, it is preferable to compress the material in a nonmagnetic compacting mold, die, etc. while subjecting it -to a magnetic field, the purpose being to align the elongated iron particles in the direction of the magnetic field, thus to obtain the optimum ratio of residual to saturation induction of the iron or other magnetic material and to maintain this ratio of B,/B; during the removal of mercury by distillation. Conveniently, the pressure used is 3,000 lbs. per square inch or higher, preferably 10,000 psi, and the impressed magnetic field has a strength of 4000 gauss or higher. This procedure also removes some of the mercury.

The mercury is preferably removed fiom the mixture by vacuum distillation at an elevated temperature, the iron antimonide layer on each particle allowing this operation to be carried out without spheroidizing the magnetic particles and degrading their magnetic characteristics. In general, the temperature of distillation is 300 C. to 400 C., the pressure less than one millimeter of mercury and the time of distillation from one to four hours. After the vacuum distillation, the magnetic material contains from about 2 to 3 percent by weight of residual mercury, which amount, it hasbeen found, cannot be substantially lowered by various alterations in the operating conditions and may be considered essentially mercury-free.

The final step in the preparation of a finished or complete magnetic structure consists of grinding up the more or less porous mass of iron, antimony and lead which remains after the vacuum distillation process and pressing it in a directionalizing magnetic field employing conventional powder metallurgy techniques, using typically a pressure of about 50,000 lbs. per sq. inch and a magnetic directionalizing field of about 4,000 gauss or more. Alternativcly, the mass remaining after the vacuum distillation process can be pressed hot at a temperature of about 350 C. and at pressures from 3,000-50,000 p.s.i., preferably 18,000 p.s.i. to flow the lead binder into position, at the same time maintaining a directionalizing field of about 4,000 gauss on the material.

t has been found that comparable BH values using a lead matrix require about A; the pressure required to attain the same 13H values in a mercury matrix. For example, when a certain lot of antimony-coated iron having a lead matrix was pressed at 350 C. under a pressure of 4,000 p.s.i., the B value obtained was 6500 gauss, the B /B value was 0.79, and the BH value was 1.9 10 gauss oersteds. When a portion of the same lot of antimony coated iron having a matrix of mercury was pressed at a pressure of 24,000 p.s.i. at 20 C., under the influence of a field having a strength of 7,000 oersteds, the B value was 7350, the B /B value was 0.82, and the BI-l value was 2.22. 10 gauss oersteds.

Shown in Table II below are the magnetic characteristics of the present material pressed into magnet form just before removal of the mercury and after the formation of the antimonide coating at 50,000 psi. compared with the same characteristics of material pressed at 50,000

'p.s.i. under the influence of a 4,000 gauss directionalizing field.

Table II Pressed in Pressed alter mercury, distillation, 50,000 p.s.i. 50,000 psi. pressure pressure From the above, it will be noted that there is at most about a 10 percent loss in magnetic characteristics in the final magnetic structure with mercury essentially removed as compared to one in which the mercury is retained.

There is provided by this invention means for providing a non-reactive matrix for protecting particles of magnetic iron and iron-cobalt material which have thereon an antimonide coating. While the invention is particularly applicable to elongated single domain magnetic particles as described herein, it will be realized that magnetic particles of other sizes having an antimonide coating can also be treated as taught herein to preserve their magnetic characteristics.

What I claim as new and desire to secure by Letters Patent of the United States is:

'1. Ina magnetic structure comprising finely divided particles of a magnetic material selected from the group consisting of iron and iron-cobalt alloy, said particles being coated with an antimonide of said material, a matrix of a material selected from the group consisting of lead and a lead-antimony alloy containing up to about 2 percent by weight of antimony there being at least about one part by weight of said matrix material to each part by weight of magnetic material.

is that of a single domain, said particles being coated with an antimonide of said material, -a matrix of a material selected from the group consisting of lead and a lead-antimony alloy containing up to about 2 percent by weight of antimony there being at least about one part by weight of said matrix to each part by weight of magnetic material.

3. The process of separating at elevated temperatures mercury from a mixture comprising mercury and finely divided magnetic particles selected from the group consisting of iron and iron-cobalt alloys having thereon an antimonide coating which is the reaction product of antimony and said particles, which includes the step of protecting said coated particles against chemical reaction which step comprises adding to the mixture comprising mercury and the magnetic particles a matrix material selected from the group consisting of lead and lead-antimony alloys containing up to about 2 percent antimony there being at least about one part by weight of said matrix to each part by weight of magnetic material.

4. The process of separating at elevated temperatures mercury from a mixture comprising mercury and finely divided magnetic particles selected from the group consisting of iron and iron-cobalt alloys, said particles being elongated and having a transverse dimension which is that of a single domain and having thereon an antimonide coating which is the reaction product of antimony and said particles, which includes the step of protecting said coated particles against chemical reaction which step comprises adding to the mixture comprising mercury and the magnetic particles a matrix material selected from the group consisting of lead and lead-antimony alloys containing up to about 2 percent antimony there being at r 6 least about one part by weight of said matrix material to each part by weight of magnetic material.

5. A method of protecting from chemical reaction finely divided particles of magnetic material selected from the group consisting of iron and iron-cobalt alloys, said particles having a coating thereon which is the reaction product of antimony and said material being elongated and having a transverse dimension which is that of a single domain which method comprises surrounding said coated particles with a matrix material selected from the group consisting of lead and lead-antimony alloys containing up to about 2 percent by weight of antimony there being at least about one part by weight of said matrix material to each part by weight of magnetic material.

References Cited in the file of this patent UNITED STATES PATENTS 2,082,362 7 Stevens June 1, 1937 2,239,144 Dean et al Apr. 22, 1941 2,563,520 Faus Aug. 7, 1951 2,601,212 Polydoroff June 17, 1952 2,825,670 Adams et al Mar. 4, 1958 2,849,312 Peterman Aug. 26, 1958 OTHER REFERENCES Paine et a1; Physical Review, November 15, 1955, pages 1055-1059.

Fine Particle Magnets, by Paine et al., page 16, pub. by General Electric, West Lynn, Mass.

Luborsky et al.: J. Applied Physics, vol. 28, pp. 344- 351, March 1957. 

1. IN A MAGNETIC STRUCTURE COMPRISING FINELY DIVIDED PARTICLES OF A MAGNETIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF IRON AND IRON-COBALT ALLOY, SAID PARTICLES BEING COATED WITH AN ANTIMONIDE OF SAID MATERIAL, A MATRIX OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF LEAD AND A LEAD-ANTIMONY ALLOY CONTAINING UP TO ABOUT 2 PERCENT BY WEIGHT OF ANTIMONY THERE BEING AT LEAST ABOUT ONE PART BY WEIGHT OF SAID MATRIX MATERIAL TO EACH PART BY WEIGHT OF MAGNETIC MATERIAL. 